Like most people say: you need to visit the National Museum in Ashgabat beforehand to understand most of what went on in Nisa or why it is important. In fact almost all artifacts are IN the national museum. What's worse is that there some columns and walls underneath the rubble in Nisa because they want them to stay hidden and protected - fair enough but not cool for you visiting at all.
The guide told me that as a kid he would just come up and play ball here. Well, basically these days it's locked up but one could still go in and destroy but there is nothing here. The columns underneath the dirt aren't interested to most folk although I would find it more interesting that seeing small mounts of dirt. After a long approach you can finally waltz around a few walls remaining but it's not so special. There are some areas where you can see the old wall, even the old columns, and I don't know if everyone gets to see the inside of one of these buildings but it was definitely the highlight of my trip. Some things inside are replicas but there are indeed ancient remains. You can get closer than in the museum and people coming to Nisa as visitors aren't going to destroy these - I think. By the way if you didn't come with a guide (which isn't possible I feel) then you'd see nothing and go back very disappointed.
In all this isn't a long visit. I probably spent less than an hour looking around the actual remains with the rest of my trip spent on the seeing Ashgabat.